Naturally occurring proteins sometimes have one or more domains, which can serve different functions. Domains can target the protein to a certain nucleic acid sequence or region of the cell. Domains also can posses an activity that is performed when the protein is targeted to a particular cellular region or nucleic acid sequence.
A fusion protein often is a protein engineered to combine a desired activity domain with a targeting or localization domain. Novel fusion proteins can be designed to carry out specific functions based on the various combinations of targeting and activity domains that can be combined. Fusion proteins are useful in laboratory, clinical and other settings.